1. Field of the Invention
The present invention relates to an audio signal processing circuit.
2. Description of the Related Art
Recently, an FM (frequency modulation) transmission circuit is used to reproduce music data recorded in a portable music reproduction device, etc., for example, by a car stereo (Japanese Patent Laid-Open Publications No. 2006-262521 and No. 2007-88657, for example.)
FIG. 5 shows an example of a commonly-used configuration of a transmission device 200 including an FM transmission circuit 300 for transmitting an audio signal. A frequency of a carrier wave in the FM transmission circuit 300 is required to be determined in consideration with a frequency of an FM radio, etc., being used in a surrounding area. Thus, firstly a user is required to set the frequency of the carrier wave in the FM transmission circuit 300. Specifically, the user operates a key (not shown) of a setting device 310 so that the frequency of the carrier wave displayed on a display screen (not shown) of the setting device 310 becomes a desirable frequency. Furthermore, after the frequency of the carrier wave is determined, the user operates the key (not shown) of the setting device 310 so that frequency data of the carrier wave is output to a microcomputer 320. The microcomputer 320 outputs the frequency data from the setting device 310, as serial data SDA synchronized with a clock signal SCL, to the FM transmission circuit 300. The FM transmission circuit 300 generates a stereo composite signal based on audio signals RIN and LIN input from a music reproduction device 330 and a carrier wave of a frequency based on the serial data SDA input from the microcomputer 320, and modulates the carrier wave by the stereo composite signal, to be output as an output signal OUT to an antenna (not shown). The resistors 400 and 410 are pull-up resistors respectively for the clock signal SCL and the serial data SDA.
In the above transmission device 200, other than the FM transmission circuit 300, the setting device 310 and the microcomputer 320 are required for setting the frequency of the carrier wave in the FM transmission circuit 300. In general, the setting device 310 includes a display screen (not shown) for displaying the frequency of the carrier wave, a driving circuit for driving the display screen, etc. The microcomputer 320 is configured on a separate chip from that on which the FM transmission circuit 300 is. Furthermore, in a common transmission device 200, for example, in a case where the user sets transmission power for the FM transmission circuit 300, there are also required the microcomputer 320, etc., as in a case of setting the frequency of the carrier wave as described above. Thus, there has been a problem that a mounting area of the transmission device 200 becomes large.